404 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



thing else remaining equal, the speed of flow must be least in that di- 

 vision of the circulatory system in which the greatest resistance is 

 encountered. It has previously been shown that the friction is greatest 

 in the capillaries, and hence, it may be gathered that these tubules 

 place the greatest resistance in the path of the circulating blood. 

 A few figures may suffice to illustrate this point. Some of the capil- 

 laries are so small that the red cells cannot enter them at all, while 

 those which possess a diameter of 5-7ju, permit their passage only after 

 they have been compressed into a shape approaching the elliptical. 

 The larger capillaries, measuring 14ju and over in diameter, allow several 

 erythrocytes to pass side by side. The average length of these tubules 

 has been estimated by Tigerstedt at 0.02 cm. Moreover, if the 

 average cross-section of a capillary is 7.5/x 2 , a capillary area of 1500 

 sq. cm. would embrace two billion capillaries, placed side by side. 

 Ordinarily, of course, the capillaries recur at distances of less than 

 0.02 cm. 



In the .arterial channels, on the other hand, the blood encounters 

 only a relatively slight resistance, so that it is able to retain practically 

 the entire pressure developed by the heart until it arrives in the arteri- 

 oles. The blood rushes through these vessels with a considerable 

 speed, but its function is in no way curtailed thereby, because a direct 

 interchange between it and the neighboring cells does not take place 

 until the capillaries proper have been reached. Here radically different 

 conditions are met with. Since the walls of these tubules consist of a 

 single layer of elongated and flattened cells which are only slightly 

 thickened in the regions of the nuclei, the tissues are brought into al- 

 most immediate relationship with the blood. The latter, moreover, 

 moves past these cells with the slowest possible speed. This is im- 

 portant, because it is essential that a sufficient time be allowed for the 

 interchange of material between the blood and the lymph bathing the 

 tissue-cells. In the veins, practically the same conditions prevail as 

 in the arteries. The nutritive interchanges having been completed 

 in the capillaries, the blood again rushes onward at a much greater 

 speed, without, however, at all equaling that of the arterial stream. 



The Determination of the Velocity of the Blood flow. As the 

 dynamical conditions in the different segments of the vascular system 

 differ considerably, it is quite impossible to employ the same method in 

 all cases. Volkmann (1850) has succeeded in obtaining approximate 

 values for the speed of the arterial flow in the following way: A U- 

 shaped glass tube of definite length and caliber is connected with the 

 artery in such a way that the blood may be made to pass either through 

 it or through a much shorter tube situated in the base of this instru- 

 ment (Fig. 218). To begin with, the tubes of this instrument which is 

 known as a hemodromometer, are filled with normal saline solution which 

 is then forced into the circulation by the entering blood. The length 

 of the U-tube being known, the speed of flow may be determined with- 



